1. Field of the Invention
The present invention relates to a method for manufacturing a backside contact of a semiconductor element, in particular, a solar cell such as a silicon based solar cell wherein the backside contact comprises a metallic layer on the backside of a substrate and the semiconductor component may have further layers on the frontside and/or on the backside of the substrate. The present invention also relates to a vacuum treatment system for manufacturing a backside contact of a semiconductor element such as silicon based solar cell.
2. Description of the Related Art
Photovoltaic technology is of great interest, since its importance is believed to increase further, due to the desire to achieve an energy supply that is independent of fossil fuels. In spite of the fact that thin film solar cell technology has been researched at an increased pace lately, silicon technology still generates the largest revenues. The reason is not only that this technology is the most mature one, but also because the most efficient solar cells can be produced with it.
In the manufacture of a silicon solar cell, so far the following steps are being performed. In a first step, a damage repair and texturing of the silicon wafer is being performed. In a second step, the emitter is produced through indiffusion of a doping, e.g. of the phosphorus donator, which deposits approximately 0.5 μm below the surface of the silicon wafer, forming an emitter layer. Simultaneously with the manufacture of the emitter SiO2 is created, which is removed again in a third step through etching. Thereafter, a SiN:H anti-reflection layer is deposited, which is performed through (PE)CVD (plasma enhanced) chemical vapor deposition, or through a reactive sputtering process. The SiN:H layer serves as a passivation layer, through the hydrogen diffusing into the silicon wafer in a subsequent firing step, or diffusing into the emitter layer, passivating voids. In a fourth step, contacts are deposited on the front side and on the backside of the wafer through silk screening by using a silver paste on the front side (the side with the emitter layer), and by using an aluminum paste on the backside as a metallic layer with cutouts, into which a silver paste is inserted as a layer that can be soldered. In a subsequent fifth layer, heating is performed (firing step), through which the contacts are hardened. Thereby, the silver is pressed through the SiN:H layer on the front side in the areas, in which it was deposited onto the SiN:H layer, and onto the silicon wafer, contacting it. As already described above, thereby a simultaneous passivation of the front side voids is performed through hydrogen indiffusion and deposition onto the voids. On the backside, a BSF (back surface field) is formed through the firing step, which also causes a passivation, thus of the voids on the backside. All this is performed through aluminum defusing into the silicon wafer, whereby an Al—Si— eutectic is formed. Eventually edge insulation can be effectuated for avoiding leakage currents, e.g. through breaking the wafers.
As previously described, the back contact of such a silicon solar cell is generally made from a metallic layer, and otherwise possibly comprises a barrier layer and a layer that can be soldered. Typically, the metallic layers of the backside contacts are currently manufactured through silk screening. Thereby, it is required for large scale application in vacuum treatment systems with continuous operation, which make the manufacture of such solar cells economical, and in which several substrates are coated simultaneously, to provide each single substrate separately with such a back contact. This means that for each single substrate a separate silk screening process has to be performed. Thereby, the throughput of such a vacuum treatment system is limited. Furthermore, special handling systems are required, in order to rotate the substrates, thereby the cost of such systems is increased and the throughput is further reduced.
Furthermore, it is a disadvantage of the backside contacts thus manufactured that the silk screening pastes, which are being used, are expensive, and the contact, which is being formed, is of poor quality, since the hardened layer is porous, and only this way a punctiform contact is present. For the metallic layer, a layer thickness of approximately 30 μm is required, whereby thin wafers can bend. This influence is gaining importance, since there is a desire to reduce the wafer thickness. The wafer thickness is thus determined as a tradeoff between cost and efficiency, wherein very thick wafers are expensive due to the material required, very thin wafers are expensive due to the complex manufacture, and the efficiency is, on the one hand, determined by a sufficiently large layer thickness for light absorption and, on the other hand, through a thickness that is small enough, so that losses due to charge carrier recombination are kept small. At present, wafer thicknesses of 200 μm to 250 μm are preferred, wherein a bending would have negative effects.
It is the object of the present invention to increase the efficiency of vacuum treatment systems in the manufacture of solar cells with metallic back contacts, and thus in particular to make a silk screening step redundant. Thus the manufacture shall be economical, in particular commercially viable, and shall be possible with a higher throughput than possible so far.
This object is accomplished through a method according to claim 1, and through the use of a vacuum deposition system for performing this method according to claim 17. Advantageous refinements of this object can be derived from the respective dependent claims.
For small batches of silicon solar cells, depositing the metallic layers in vacuum was already suggested by U.S. Pat. No. 7,071,081 B2. However, not in a inline vacuum deposition system, and exclusively for generating a BSF (back surface field), wherein initially a metallic layer made from aluminum is deposited through vapor deposition, or through sputter deposition, said layer is then sintered and deposited with a group V-element. These three process steps have to be performed in three different apparatuses. Therefore, also this deposition method can not be economically applied in vacuum deposition apparatuses with inline operation, and furthermore, the actual metal backside contact still has to be applied through silk screening.
From U.S. Pat. No. 7,071,018 B2, it is furthermore known that silicon solar cells have been realized on a lab scale, wherein an aluminum layer with a layer thickness of greater or equal 2 μm was deposited through PVD on a thin dielectric SiO2 or SiN layer. The SiO2— or SiN layer, on the one hand, facilitates the buildup of the BSF and, on the other hand, avoids a doping diffusion. This method, however, is not suited for a commercial, cost efficient production, since silicon wafers are necessary for this purpose, which are manufactured according to the floating zone method.
Through the method according to the invention, a silk screening step for depositing the backside contacts is not necessary any more, and the vacuum is not interrupted, so that an undesired oxide formation, and therefore the subsequent cleaning step are avoided.